First-principles investigation of p-type Be3NX3 (X = F, Cl, Br) perovskites for optoelectronic and thermoelectric energy-harvesting applications

Lead-free halide perovskites have attracted attention for optoelectronic and photovoltaic applications due to their tunable properties. However, Pb-free composition alone does not guarantee environmental safety, and constituent-element risks must be carefully considered. Here, first-principles calculations are performed for cubic Be 3 NX 3 (X = F, Cl, Br) halide anti-perovskites, focusing on their structural, mechanical, electronic, optical, thermoelectric, and dynamical properties. Geometry optimization, formation-energy analysis, tolerance factors, and simulated XRD patterns confirm the stability of the Pm-3m phases, with lattice expansion and weakened bonding from F to Br. All compounds satisfy the Born mechanical-stability criteria and exhibit predominantly brittle behavior, while Be 3 NF 3 shows the highest stiffness, hardness, and fracture resistance. HSE06 calculations predict band gaps of 6.987, 2.648, and 1.456 eV for Be 3 NF 3 , Be 3 NCl 3 , and Be 3 NBr 3 , respectively, revealing a transition from a wide-gap ultraviolet-active material to visible-light-responsive semiconductors. The heavier halides exhibit stronger dielectric screening, lower exciton-binding energies, and improved visible-light absorption. Low lattice thermal conductivities of 1.62–1.46 W/m.K at 300 K support favorable thermal transport. Positive Seebeck and Hall coefficients confirm p-type conduction, while ZT reaches 2.68, 2.09, and 2.15 at 900 K. AIMD simulations further confirm room-temperature and 900 K dynamical stability, highlighting Be 3 NX 3 as tunable candidates for optoelectronic and high-temperature thermoelectric applications.